Saturday, May 22, 2010

Interpolation wars

15 days after the release, one issue in the packaging and some glitches on linkicc. Still no big bugs. At least the reported  ones are not such big to need a 2.1 release... yet.

Little CMS releases have been historically dropped each 6 months, so 2.1 should come on October-November more or less. I have a talk on the ICC DevCon on November, so it would be nice to have 2.1 released to at that time.

Otherwise the git is alive and I'm periodically committing  fixes to it. If you are using Little CMS code and don't have dependencies on linux distributions, I would recommend to use git code if possible.

One of those small glitches found is a weird error that was reported by a HP folk days ago. He was seeing
small differences on a CMYK profile when upgrading from 1.19 to 2.0

So, after investigation, I found the reason of those differences: Tetrahedral interpolation being used in 2.0 and Trilinear interpolation in 1.19. Tetrahedral was patented time ago, but now the patent has expired.

In fact, I did see such issue many years ago. On LUT elements being indexed by Lab colorspace, Tetrahedral does not work well. I suspect that's because Luma is uncentered (L is on one axis)

First thing to discard was a code bug. So I tried Max Derhak's SampleICC. To my astonishment, SampleICC is also using trilinear by default. Tried to modify Max code to do the interpolation as tetrahedral and... bingo! the same "bad" results as Little CMS. Up to four decimals. So here we go, the "bug" is in the interpolation algorithm. I checked PhotoShop CS4. It seems to be also using trilinear as well.

Now I have committed those changes to git. Note however, that your are not going to notice any difference but in very few profiles, and even in this case, maximum difference is about 2-3 digital counts (0.5 dE)

1 comment:

  1. So, what are the changes you have implemented? Allowing, heuristically, trilinear interpolation?